Field of the Invention
One disclosed aspect of the embodiments relates to an image forming apparatus that performs optical writing by using a laser beam, such as a laser beam printer (LBP), a digital copying machine, and a digital facsimile (FAX).
Description of the Related Art
An electrophotographic image forming apparatus includes an optical scanning unit, or scanner, for exposing a photosensitive member. The optical scanner emits laser light based on image data, reflects the laser light with a rotating polygonal mirror, and passes the laser light through a scanning lens to irradiate and expose the photosensitive member. The rotating polygonal mirror is rotated to move a spot of the laser light formed on a surface of the photosensitive member for the purpose of scanning, thereby forming a latent image on the photosensitive member.
The scanning lens is a lens having an fθ characteristic. The fθ characteristic refers to an optical characteristic of the lens in forming a laser light image on the surface of the photosensitive member to move over the surface of the photosensitive member at a constant speed when the rotating polygonal mirror is rotating at a constant angular speed. By using the scanning lens having the fθ characteristic appropriate exposure can be achieved.
The scanning lens having such an fθ characteristic comes in a relatively large size and is costly. For the purpose of miniaturization and cost reduction of the image forming apparatus, disuse of the scanning lens itself or use of a scanning lens having no fθ characteristic has been contemplated.
Japanese Patent Application Laid-Open No. 58-125064 discusses an electrical correction method for changing an image clock frequency during a scan so that even if the spot of the laser light on the surface of the photosensitive member does not move over the surface of the photosensitive member at a constant speed, dots having a constant width are formed on the surface of the photosensitive member.
In order to suppress image defects due to uneven charging, Japanese Patent Application Laid-Open No. 8-171260 discusses an image forming apparatus that not only exposes an image part where toner adheres to, but also performs post-exposure on a non-image part where toner does not adhere to. Japanese Patent Application Laid-Open No. 2012-189886 discusses an image forming apparatus that includes a plurality of image forming stations and forms a color image, wherein the image forming stations use a common charging voltage and developing voltage. Japanese Patent Application Laid-Open No. 2012-189886 discusses performing exposure on a non-image part with a small amount of light to maintain an appropriate non-image part potential if photosensitive drums of the respective image forming stations have different film thicknesses.
However, it is not clear how to perform the weak exposure on a non-image part as discussed in Japanese Patent Application Laid-Open Nos. 8-171260 and 2012-189886 with a configuration not using a scanning lens having an fθ characteristic.